get off with (someone)
1. Literally, to disembark (e.g., from a vehicle, elevator, etc.) with someone. I ran into George on the bus and decided to get off with him at his stop in town.
2. slang To have an amorous or sexual encounter with someone. Primarily heard in UK. We weren't at the party for more than 30 minutes when Jake got off with some guy he'd just met. I heard Janet got off with one of her co-workers.